This invention deals with the preparation of high purity basic lead carbonate and high purity normal lead carbonate in a single, multi-step process. The process involves a double precipitation reaction whereby the desired lead carbonate compounds are obtained from a lead acetate solution.
In the past, basic lead carbonate has been produced by a number of processes. In one process, commonly known as the Dutch process, metallic lead is cast in small perforated plates or buckles, which are placed in earthenware pots over a weak solution of acetic acid. The pots are stacked in layers in tanbark, each tier being supported by wood boards. The tiers are built up to as many as ten layers and the whole stack is then boarded up. Fermentation of the tanbark generates considerable heat and carbon dioxide. The heat volatilizes the acetic acid, causing it to react with the lead to form the lead acetate, which, in turn, is carbonated by the carbon dioxide to form the basic lead carbonate. After 100 to 120 days, the stacks are dismantled and the white powder is removed and ground in water, forming a water pulp. The water pulp may be filtered and dried to produce a dry basic lead carbonate powder, or it may be converted to a basic lead carbonate/oil paste by the addition of linseed oil. Because of the great affinity of basic lead carbonate for linseed oil, the oil will displace the water from the basic lead carbonate. The lead-in-oil paste is finished by passing it over a roller mill.
In a second process, generally known as the Carter process, powdered lead is suspended in water in large revolving wood cylinders, acetic acid is added, and carbon dioxide is passed into the cylinder. The reaction is completed in 10 to 12 days. This process permits better control than the older Dutch process and the finished product can be made with a wide range of physical properties.
Another process, commonly referred to as the Euston process, starts with feathered or mossy lead made by running molten refined lead metal into water. The feathered lead is then oxidized by air in a solution of normal lead acetate into which carbon dioxide is introduced. The basic lead carbonate forms as a precipitate and the lead acetate solution is recovered by filtration and re-used.
Yet another process is an electrolytic process, which makes use of a concrete cell divided by a porous membrane. In one portion of the cell, a lead anode is suspended in a solution of sodium acetate, and in the other portion of the cell an iron cathode is immersed in a basic sodium carbonate solution. The passage of the electric current removes the lead from the anode as lead acetate, which forms a precipitate of basic lead carbonate in the anolyte.
Another process, generally referred to as the Thompson-Stewart process, produces a basic lead carbonate of high quality and very fine particle size. Finely divided litharge, lead, or a mixture of both, is suspended as a slurry in water. Acetic acid is added, the material is aerated, and carbon dioxide is passed through. White lead of high basicity and fine particle size is produced in the process. See, e.g., U.S. Pat. No. 2,218,940 and Kirk-Othmer Encyclopedia of Chemical Technology, Volume 10, pp. 614-616 (1953).
Various processes for the preparation of white lead are described, e.g., in U.S. Pat. Nos. 428,017, 720,670, 1,349,334, 1,587,623, 1,720,196, 1,916,302, 4,118,219, Great Britain patent specification Nos. 226,689, 495,051 and the like.
Similarly, normal lead carbonate has been produced by a number of processes. See, e.g., U.S. Pat. Nos. 1,587,623, 1,916,302, 3,883,348 and the like.
Certain of these processes are known to generally disclose the production of either basic or normal lead carbonate. See, e.g., U.S. Pat. Nos. 1,587,623 and 1,916,302. However, such processes result in the precipitation of an admixture of basic and normal lead carbonate and are thus not suitable for the production of both a high purity basic lead carbonate product and a high purity normal lead carbonate product.
The ability to produce a high purity basic lead carbonate product and a high purity normal lead carbonate product in a single process and from the same starting material would be highly desirable from a commercial standpoint. Basic lead carbonate is capable of use as a pigment or as a heat stabilizer for flexible polyvinylchloride manufacture. Normal lead carbonate is useful as a starting material for the formation of various other lead compounds. The ability to obtain both these compounds in commercially useful form by a single process would also provide the opportunity for savings in equipment expenditures.
A single, multi-step process which produces high purity basic and normal lead carbonate products would have significant applicability in connection with the preparation of pure lead monoxide from impure lead sulfate bearing materials, particularly impure lead sulfate bearing materials such as recycled battery mud. One such process for the preparation of pure lead monoxide from recycled battery mud is described in U.S. patent application Ser. No. 126,625, entitled "Production of Lead Monoxide from Lead Sulfate with Acetic Acid", and filed Mar. 3, 1980, by Eugene Striffler Jr., et al, now U.S. Pat. No. 4,269,811, and is commonly assigned to the assignee of the present application. In such a process, pure lead monoxide is prepared in a multi-step process which comprises:
(a) reacting a lead sulfate-bearing material with an ammonium carbonate solution to convert lead sulfate to lead carbonate; PA1 (b) decomposing the lead carbonate to form impure lead monoxide; PA1 (c) reacting the impure lead monoxide with acetic acid to form a lead acetate solution; PA1 (d) contacting the lead acetate solution with carbon dioxide to produce insoluble lead carbonate; and PA1 (e) decomposing the lead carbonate to form substantially pure lead monoxide.
Any lead dioxide present in the lead bearing material may also be decomposed along with lead carbonate in step (b) of the process to produce additional lead monoxide. Alternatively, such lead dioxide may be treated with acetic acid in step (c) together with a reducing agent to simultaneously decompose the lead dioxide and form additional lead acetate.
The process may be conducted in a continuous manner. In the continuous mode, the by-products of reactions occurring in the process are used to form the reagents used in the various steps of the process. More particularly, carbon dioxide formed as a by-product of the decomposition of lead carbonate in step (b) can be separated and combined with ammonia to produce the ammonium carbonate solution used in step (a). The carbon dioxide formed as a by-product of the decomposition of lead carbonate in step (e) can be recycled for use in step (d). The acetic acid produced as a by-product in step (d) can be recycled for use in step (c).
In addition, lead chemicals such as lead chromate, lead arsenate, and lead tungstate can be prepared by precipitation from the lead acetate solution formed in step (c) with appropriate reagents and separating the so-formed lead chemicals from the remaining solution.
A single, multiple-step process for the conversion of lead acetate solution to high purity basic and normal lead carbonate products could serve to replace step (d) above, thus providing the versatility to produce two commercially useful products, i.e., both types of lead carbonate, in the same processing equipment and from the same lead acetate starting material.
Thus, the search has gone on for a single, multi-step process for the double precipitation of high purity basic lead carbonate and high purity normal lead carbonate from a single starting material.